The kind of thoughts that keep a person from falling asleep can arise in either half of the brain. Therefore, a person being prevented from sleeping solely by such thoughts would be able to fall asleep by closing the eyes and counting sheep, because this activity fully occupies the left half of the brain with imagining sheep, thereby excluding the sleep-preventing thoughts.
What this question is testing
Your task
Find the assumption the argument requires in order for its conclusion to hold.
Common trap
Answers that would help the argument but aren't strictly required (sufficient, not necessary).
Winning move
Negate each choice — the right one breaks the argument when negated.
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